The secret life of reality

INTRODUCTION: One thing that is interesting here is the is the durability of Penrose’s ideas. These were often decried by famous-for-a-day scoffers in the 1990s, but are now still referred to, or revived, when some of the original scoffers may be long forgotten. Here a popular science magazine reviews experiments relative to Penrose’s idea for quantum gravity.

Physics has for a long time tried to resolve the incompatibility between relativity theory and quantum theory. The superposition of possible states identified in the quantum wave needs to be rendered compatible with gravitational theory. Roger Penrose has promoted the idea that experiments involving gravity and quantum theory might resolve this problem.

Gravity, superpositions & time

Our understanding of gravity is based on general relativity which states that massive objects produce curvature in spacetime which in everyday life translates into the attractive force of the Earth and other massive objects. This doesn’t mix well with quantum superposition because some quanta have mass and therefore presumably a small measure of gravitational attraction, and given that more than one version of a quanta is in superposition, these superpositions could distort spacetime and exert a gravitational force on one another.

There is also a problem with time. In relativity, each particle has its own frame of reference in both space and time. With a superposition of particles, each superposition may have a slightly different time, or in other words, age at a slightly different rate, implying a problem when the superpositions of the wave function collapses to a single particle.

A new generation of experiments

A possible solution is seen in a new generation of experiments that look for gravitationally induced decoherence. Such experiments will involve quanta in superposition. It has been discovered that large numbers of atoms cannot maintain superposition. It can still be maintained with molecules of up to 800 atoms, but the more massive the system becomes the quicker it reaches decoherence. Technical advances have made it easier to shield systems from external factors that promote decoherence, and thus concentrate on studying the influence of gravity.

Cisco Gooding and Bill Unruh, at the University of British Columbia, are planning to study superpositions of an atom taking different paths through an interferometer and the influence of time when the superposition collapses at the end of the experiment. Another suggested approach involves one-atom clocks in a superposition of different heights above the Earth where time would move at different rates because of the difference in the strength of the Earth’s gravitational field.

Another approach involves mirrored cantilevers existing in a superposition of configurations, which could collapse back into a single configuration. The superpositions need to last long enough to allow the investigation of the decohering effects of gravity. What has changed in recent years is that more experiments in this area can be done, or look like becoming doable, within the near term. Thus over a 10 year timescale it might be possible to establish the role of gravity in superpositions.

NOTE: What is not mentioned in this article is that Penrose suggested that the gravitationally induced collapse of the wave function within the brain could connect it to consciousness or understanding existing at the level of fundamental spacetime geometry.